1. Field of the Invention
This invention relates to an improved laser beam useful in material surface processing and an apparatus for producing the improved laser beam. The improved laser beam of the present invention comprises a power distribution that is more intense at the outer regions than in the central region. The invention includes an apparatus comprising optical elements aligned to focus and/or shape a laser beam into a novel profile useful in material surface processing. The laser beam and apparatus of the present invention are useful for producing Laser Induced Surface Improvements, hereinafter referred to as "LISI".
2. Description of the Prior Art
Laser beams have been used to treat and/or process a workpiece, such as a piece of metal. Prior art devices employing laser beams for materials processing have comprised a beam delivery system and a movement system for moving the workpiece relative to the beam. The process effect that the beam has upon the workpiece is a function of many variables, including but not limited to, power distribution and beam shape. Uniform processing of the workpiece surface by the beam is normally desired.
One family of prior art beam processing devices have the disadvantage of nonuniform surface processing because the beams used by such devices have Gaussian or normal power distributions. Jones et al., "Laser-beam Analysis Pinpoints Critical Parameters", Laser Focus World (January 1993). Another type of prior art laser beam has an equalized or uniform power distribution across the cross sectional area of the beam. Such a beam is described in U.S. Pat. No. 5,124,993. Such an equalized power distribution can result in less energy deposition at the outer regions of the laser beam irradiation zone due to increased heat transfer in these regions.
This processing nonuniformity results in surface irregularities in the processed workpiece and nonuniform processing depth under the surface area of the workpiece irradiated by the beam. Prior art beam configurations are typically circular. The power density of such circular beams is greatest at the center, thereby resulting in a maximum energy deposition at the center of the beam. The power distribution of typical prior art beams is shown in FIG. 1A.
In LISI processes, where the power at point A is constant as a function of time, the energy deposition at point A, ED.sub.A, is proportional to the product of the power at point A, P.sub.A, and the length of time, T, that point A was irradiated as shown. Where the power distribution is spatially nonuniform, the resulting energy deposition distribution is proportionally nonuniform.
Another drawback of the Gaussian power distribution of prior art beams is that the power density and corresponding energy deposition decline as a function of radial distance from the beam center. Accordingly, workpieces processed with such prior art beams are overprocessed in the central beam region and underprocessed in the outer beam regions. The depth of processing is proportional to the power distribution. The processed region is referred to herein as "the melt region". A typical melt region of a prior art beam having a Gaussian distribution is shown in FIG. 1 B. The temperature history resulting from the melt region profile depicted in FIG. 1 B and the power distribution in FIG. 1 A, is shown in FIG. 1C.
Prior art processing methods employ beam overlap during processing of the workpiece to increase the processing depth of the underprocessed outer regions. For LISI processes, surface characteristics such as smoothness and homogeneity of composition are important, particularly for hardness and corrosion resistant properties. The beam overlap required with prior art beam processing methods results in nonuniformities in processing depth, composition and surface profile. All of these nonuniformities are undesirable.
In LISI processes employing the deposition of a precursor material on the workpiece prior to irradiation, the low power densities in the outer region of the circular beam can result in the melting and removal of the precursor material before it is incorporated into the substrate melt. This can result in nonprocessed irradiated regions.
The efficiency of beam processing systems is a function of many variables, including but not limited to beam shape, size and power distribution. Prior art beam processing devices have the disadvantage of inefficient processing rates, resulting in part from, small beam area, nonuniform power distribution and circular beam shape.
The present invention offers the advantages of (a) improved power distribution, (b) improved surface quality, and (c) improved surface processing rate. These advantages are achieved through the use of a beam shaping and focusing apparatus that produces an improved laser beam with a novel power distribution that is increased at the outer regions relative to the central region. Improved surface quality results from the improved power distribution of the present invention. The improved processing rate results from the larger, noncircular beam surface area, and the more uniform power distribution.